- Author: Jim Downer
As we continue an historic drought in California, trees all over Southern California are showing symptoms of stress, and in some cases have died due to drought and pests that attack drought stressed trees. Despite recent and significant rainstorms, it is likely that trees will again be stressed later in the irrigation season. Trees and other woody plants in nurseries also bear the burden of drought stress as irrigation waters become increasingly salty because of drought effects on wells and reliance on stored water in reservoirs continues.
The Polyphagous shot hole borer is a serious pest of nursery and landscape trees and threatens many species. As such, counties adjacent to Los Angeles are at risk of invasion by this new exotic pest and we are watching and monitoring trees for possible extant new detections. The drought has stressed many of the signature trees that could be attacked by PSHB and these trees are often attacked by native or established pests. In some cases these beetles closely resemble PSHB and cause symtpoms of bleeding and in some cases cambial discoloration and gallery formation similar to that of PSHB.
On coast live oak (Quercus agrifolia) there are two look alike beetles the western oak bark beetle (Pseudopityophthorus pubipenis) and the Oak ambrosia beetle (Monarthrum scutellare). The western oak bark beetle is common in Southern California and the ambrosia beetle more common in the northern half of the state but both have been detected throughout California. As with many beetle/tree interactions both of these are attracted to drought stressed trees. When attacked coast live oak bleeds heavily and these insects often attack in larger numbers uniformly along the main stems or trunk. Examination of the cambium of Oak Bark Beetle attacked trees reveals small galleries and little or no necrotic tissue.
Prunus spp such as peach, nectarine, plum and apricot are common in back yards and retail nurseries. This year we are seeing much increased attack of these trees by the European fruit borer or shot hole borer (Scolytus rugulosus) a beetle very similar in stature to PSHB. The insect is the same size as PSHB and makes numerous holes and causes excessive bleeding in Prunus. This insect also seems to carry a fungal pathogen resulting in cambial necrosis.
One other insect that could be mistaken for PSHB is the walnut twig borer which vectors Thousand Cankers Disease of walnuts (both native and English). This beetle (Pityophthorus juglandis) carries with it a fungus Geosmithia morbida, that causes extensive cambial necrosis, and ultimately twig and branch dieback results from its presence.
- Author: Ben Faber
Polyphagous Shot Hole Borer can go to over 100 and counting hosts, but the Fusarium that the ambrosia beetle carries can not reproduce on all tree species. It is this fungus that ultimately kills the tree. There are now though thirty-seven hosts on which the beetle will attack and the Fusarium will grow. Here is the current list, 14 of which are California natives:
Known Suitable Reproductive Host Trees:
*Native species to California
- Author: Sabrina Drill
As discussed in previous issues of this newsletter, polyphagous shot hole borer (PSHB; Euwallacea sp.) is a new pest/fungal complex attacking a wide variety of host trees in Southern California, from avocado to common residential and street trees and native oaks and riparian species. The PSHB is morphologically identical to the tea shot hole borer, E. fornicatus, but a genetic analysis confirmed that this is a new species of ambrosia beetle. PSHB has been found to carry several symbiotic fungi, including new fungal species Fusarium euwallacea, andan undescribed Graphium species.It was first identified in 2003 in Whittier Narrows, an undeveloped riparian area in Los Angeles County, and was officially first linked to tree injury and mortality in a residential avocado in 2012. Since then, the pest complex, also referred to as Fusarium die-back, has spread throughout Southern California and is now present in Los Angeles, Orange, San Bernardino, Western Riverside, and San Diego counties. The population in San Diego County is a different genotype from that found in the rest of the region, possibly indicating a separate introduction. The pest/disease complex has caused significant impacts to the avocado industry in Israel, and is now spreading in commercial groves from Escondido to Fallbrook. It has caused injury and mortality of hundreds of ornamental trees in the urban areas, impacting roadways, botanical gardens, parks, and private residences, and impacts all major native riparian trees species. The official list of reproductive hosts, meaning trees that can support growth and reproduction of the beetles and fungi, includes 35 species with several having been confirmed in just the past few months.
The plant pathology, IPM, environmental horticulture, and natural resource management teams working on this pest at UCR and UC Cooperative Extension in San Diego, Orange, Los Angeles, and Ventura Counties have put together several useful tools that can help you manage this pest. These include a new field identification card set, a decision tree for tree removal, a guide to managing infested plant materials, and information about how to report an infested tree and how to collect samples for lab identification, as well as a continuously updated web-based map. You can find these at www.pshb.org.
To determine if your trees are affected by this pest:
- Look for a small (tip of a ball point pen) round entry/exit hole surrounded by wet discoloration of the outer bark
- Follow the gallery to look for the beetle (may or may not be present)
- Look for other hosts (Castor bean, sycamore, maple, coast live oak, goldenrain, liquidambar) showing symptoms of the beetle/disease
- Report suspect tree infestations to firstname.lastname@example.org with the following information:
o Your contact info (name, city, phone number, email)
o Suspect tree species
o Description of suspect tree's location (and/or GPS coordinates)
o Description of suspect tree's symptoms
o Photos of suspect tree and close-up photos of symptoms
- If symptom photos and descriptions indicate it might be PSHB/FDB a field assessment may be needed or a sample can be submitted following detailed directions on how to collect and submit a sample for fungal confirmation at pshb.org.
To protect your trees, avoid movement of infested firewood and chipped material out of infested areas. Infested material should be chipped to under 1”, wrapped in clear plastic, and solarized on site for up to 6 mos (depending upon environmental conditions). Research is ongoing in the use of insecticides and fungicides, as well as exploration of the use of endophytic bacteria as a biocontrol agent. If you suspect that you have found this beetle or seen symptoms of the Fusarium dieback on your tree please contact the Ventura CE office, your pest control advisor or qualified arborist, or contact UC Extension Plant Pathologist Dr. Akif Eskalen by at email@example.com. For more information visit www.pshb.org or http://eskalenlab.ucr.edu/avocado.html.
- Author: Gary Bender
The polyphagous shothole borers (Euwallacea sp.) that spread fungal diseases (Fusarium sp. and possibly Graphium sp.) to susceptible trees in Los Angeles County have now been found in mid and northern Orange County and western San Bernardino County. Sick and dying trees are being cut down and shredded or chipped. A lot of different species of trees are affected, including avocado, box, elder, castor bean, coast live oak, Engelmann oak, sycamore, bigleaf maple, California bay laurel, white alder, olive, peach persimmon, goldenrain, mimosa, liquid amber and wisteria vine.
Why is this important to growers in San Diego, Riverside, Ventura and Santa Barbara counties? Because growers in San Diego County (and probably other coastal counties) are being offered free shipping by the waste disposal companies of wood chips and free spreading of the mulch in their groves. What a deal!! But wait a minute!
The problem lies in that the material I have seen is either not composted or poorly composted, because it heats up in the grove after delivery and starts steaming. This means that freshly shredded or chipped trees could very likely be spreading the borers right into their groves!
Growers should ask themselves “why are these trees in Los Angeles being cut down in the first place?” Trees are being pruned and cut down for a variety of reasons, but now that we have a new pest for which there is no control, we have to be very cautious about what we bring into our groves. Other problems that could be brought into groves include Phytophthora root rot and trunk cankers, oak root fungus, Dothierella cankers, and Asian citrus psyllid.
Growers should insist that only correctly composted mulch be brought into their groves. During the composting process the piles should be turned at least five times to allow the material on the outside of the pile to be turned into the middle for correct heating of the entire pile.
- Author: Akif Eskalen
This is an update on our recent findings on PSHB/Fusarium dieback. As of October 4, 2013, PSHB/Fusarium dieback was detected in Glendora in northern Los Angeles county and Laguna Niguel in southern Orange County. The infestation in Laguna Niguel Regional park appears to have arrived there within the last week. We inspected the South coast research station last week, and it was still negative. Please find attached recent distribution map.